Customise LED strip from a plant lamp

Hey,

I have three LED strips from a plant lamp I‘ve bought some time ago. On these strips are red and blue LEDs (so 3 solder connections on these: red, blue and ground).

From the power adapter I know that the lamp has a power output of 30 watts and 5V. So each strip requires a current of 2A.

The question I have is: can I simply wire these 3 strips in parallel to each other and then control them via transistor and Arduino?

Thanks in advance :slight_smile:

Ps: Posted this in general electronics because its not a Led specific question

because its not a Led specific question

It is. The answer depends on the nature of these LED strips.

At the moment the information does not make much sense because while it is not impossible to have a strip with “3 solder connections on these: red, blue and ground” I have never seen one. They are normally power and red, green and blue that have to be grounded with a transistor.

Can you post some pictures of what you have along with closeups of the connected end of the strip.

Oh yeah, sorry for that.

The red LEDs and the blue LEDs are like parallel to each other.
The actual control unit for the lamp has two buttons: on/off, color selector (red, blue or both)

I‘ve attached the pictures of the metal, pcb-like strip.

The red LEDs and the blue LEDs are like parallel to each other.

Well sort of each LED has a resistor in series and then the LED / resistor combination are paralleled up.

antonright:
On these strips are red and blue LEDs (so 3 solder connections on these: red, blue and ground).

So like I suspected red, blue and positive, not ground.

So use it like this Usage | RGB LED Strips | Adafruit Learning System
Only connect the positive to 5V not 12, and do not connect the 5V from you power supply where it is shown on the Vin but to the 5V pin.

Yes you can put the strips in parallel on the FETs. Note you need an external power supply as you can't power it from the USB power on the Arduino.

So each strip requires a current of 2A.

Are you sure the strip takes all the 2A the power supply is capable of? If so that will be a 6A power supply you need. But you could just use your existing power supplies one for each strip for the 5V line into the strip and make common all the grounds for each power supply and the Arduino.

Note you need an external power supply as you can't power it from the USB power on the Arduino.

My plan is to use the existing power supply with 5v and 30 watts.

I've made the layout in Fritzing. Is this correct how it's done? (I especially don't know how the two grounds should be connected)

The circuit is OK, except that in practice 2A is too much for a breadboard and could melt/damage it. So test the circuit on breadboard but do not connect the strip. Instead, connect a couple of ordinary 5mm leds with series resistors for the test. When testing is complete, solder the circuit up on stripboard. For the sections of the stripboard tracks that carry the high current, put a thick layer of solder, enough to cover the stripboard holes.

Your Fritzing picture shows npn transistors. These can work, and if you have them already, and they will switch only 2A, use them. But if you need to purchase the transistors, buy logic-level n-channel mosfets instead of npn transistors or darlington transistors.

Since I’ve not that much experience in mosfets, could you recommend one?

And what resistor do I have to use from the mosfet to the Arduino?

PaulRB:
Your Fritzing picture shows NPN transistors. These can work, and if you have them already, and they will switch only 2A, use them.

But they wouldn't. Not without a decent heatsink as they will not saturate to pass 2 A with 20 mA base current. 200 or 300 mA perhaps. :roll_eyes:

antonright:
Since I’ve not that much experience in mosfets, could you recommend one?
And what resistor do I have to use from the mosfet to the Arduino?

A couple of these inexpensive modules would solve your problem entirely:

How do I use these? I think PWM and Ground to Arduino, but what about the other side?

How do I use these?

Just like the link I posted in reply #3.
The PWM should be applied to either the base of the transistor, through a 510R resistor, or if you are using a FET directly to the gate of the FET. With the source going to ground and the drain to the LED on the strip.

antonright:
How do I use these? I think PWM and Ground to Arduino, but what about the other side?

Heh, heh, my apologies!

I have a couple of these on order from Aliexpress, ordered 31st August.

It is barely three weeks, the slow boat from Wuhan may take another month for all I know. :grinning:

The black PCB prevents me from figuring out the circuit until I actually have them in hand, so whether (as I would expect) it includes the series resistor for the optocoupler LED or not, and which terminal of the three is which for the output side, I do not know.

The implication is that if you were to order a couple, then by the time you get them I should have mine and could provide compete details! :sunglasses:

IRLZ44N, STP60NF06L... there are many suitable models, but make sure it has logic level gate and is TO-220 package for ease of soldering.

Hey :slight_smile:

I've got this LED strip (originally from a lamp) which I connected (on a trial basis) to this kind of relais and to a 5V, 1A power supply.
I connected both "-" lines (Red and Blue) to the OUT- and tested this setup with PWM on an Arduino.

I got around 60 (maybe more?) as frequency on analogWrite by comparison of light intensity with the original lamp.

My question is: how can I calculate the exact amount of current drawn by one single LED, since I don't want to overstress them by giving them too much current.

From what I know, I (think I) can calculate the current drawn by the strip while on PWM with:

1A/256 = 0,0039A (per PWM point)
60*0,0039A = 0,234 A (for the whole strip)
and 0,234/ 20 = 0,0117A ~ 10mA per LED

Is this correct?

Thanks in advance :slight_smile:

And am I right that the voltage over the LEDs is 0,7V?

The resistor in front of the LEDs is 430 Ohms and the assumed current is 10mA.

So with Kirchhoff‘s Voltage Law, the voltage of the LEDs is: 5V - 430 Ohms * 10mA = 0,7V

I got around 60 (maybe more?) as frequency on analogWrite by comparison of light intensity with the original lamp.

Not at all sure what that means.

Did you use a transistor when you drove it with PWM? The description of what you did made little sense.

From the look of your photo you have a 430R resistor in each LED. However that is not enough to calculate the current. You also need to know the voltage drop across that LED when it passes a certain current. However that sort of resistor will let you drive approximately at 10mA if you drive it with 5V. Normally these sort of strips are run from 12V. But you do need a transistor Or you will damage your Arduino.

And am I right that the voltage over the LEDs is 0,7V?

No. It depends on the colour and the stuff they are made of.

So with Kirchhoff's Voltage Law, the voltage of the LEDs is: 5V - 430 Ohms * 10mA = 0,7V

No that is neither Kirchhoff's nor a version of ohms law which is what you should be using.
You can’t subtract ohms from volts, it makes no sense.

Not at all sure what that means.

Sorry, it's the duty cycle in analogWrite()

Did you use a transistor when you drove it with PWM?

I used a MOSFET trigger switch (see annex picture)

PWM connects to your PWM pin on the arduino, ground to the arduino ground.
V-Out + goes to + on the strip and V-Out - goes to blue AND red on the strip.

Normally these sort of strips are run from 12V.

I'm pretty sure that this strip runs on 5V since the power supply provides 5V.

No that is neither Kirchhoff's nor a version of ohms law which is what you should be using.

Well Kirchhoff's second law states that "the algebraic sum of all the voltages around any closed loop in a circuit is equal to zero"
So the power supply provides 5V, the strip has the LEDs parallel to each other, so you could define one LED with resistor (430 Ohms) as one closed loop.
So the voltage over the LED is calculated by subtracting the product of resistance of the resistor (430 Ohms) and the current over the LED (and thus the resistor since they are in line) from the voltage fro the power supply.

5V- (430 Ohms* 10 milliamperes) = 0,7V

MOSFET Trigger Switch.PNG

MOSFET Trigger Switch.PNG

@antonright

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I was typing a long answer when these were merges and I lost it all when I hit post.